When using any consumable power source, a determination of the amount of operating time left before refueling is a consistent problem. For metal hydride fuel storage systems, this problem becomes especially difficult. Metal hydrides are often used to store fuels, such as hydrogen, in conjunction with fuel cells for electrochemical generation of power. Hydrogen is absorbed into a metal alloy, creating a hydride of the alloy.
Measuring the weight of the fluid stored within metal hydrides is error prone as the fluid weight is low compared to the overall weight of the system. The error in weight-based fuel gauging increases dramatically as the amount of fuel in a system lowers, leading to great uncertainty in a weight-based assessment.
In powder-based hydride systems, some try to correlate internal pressure to a state of charge. This method does not work well as hydrides are designed to operate at a constant pressure until they are nearly fully discharged. Also, this constant pressure of operation is highly correlated with environmental temperature. Therefore, any pressure-based measurement of a hydride system is a better indicator of the system temperature than it is of the system state of charge. In addition, conventional hydride systems based on powders can pack and therefore induce large strains on an enclosure, confounding the determination as to whether any hydrogen remains in the enclosure.